# Lesson 2. Automate Data Tasks With Loops in Python

## Learning Objectives

After completing this chapter, you will be able to:

• Automate tasks using data structures such as lists, numpy arrays, and pandas dataframe.
• Add the results of a loop to a new list.

As you have already learned, loops are very useful for removing repetition in your code. As such, they are great for automating tasks that you want to run on multiple values or data structures. Explore the examples below to see how you can automate tasks using data structures such as lists, numpy arrays, and pandas dataframe.

## Automate Calculations on Values in Lists

Recall that in the lessons on variables and lists, you learned how to run calculations on individual variables to convert the units, using average monthly precipitation values for Boulder, Colorado, provided by the U.S. National Oceanic and Atmospheric Administration (NOAA).

MonthPrecipitation (inches)
Jan0.70
Feb0.75
Mar1.85
Apr2.93
May3.05
June2.02
July1.93
Aug1.62
Sept1.84
Oct1.31
Nov1.39
Dec0.84

After you converted each variable, you then manually created a list that contained the recalculated values.

Using a loop, you can automate this task, so that you recalculate each value in an existing list.

### Create List of Values For Loop

Begin by creating the list upon which your loop will execute.

# Create list of average monthly precip (inches) in Boulder, CO
avg_monthly_precip_in = [0.70,  0.75, 1.85, 2.93, 3.05, 2.02,
1.93, 1.62, 1.84, 1.31, 1.39, 0.84]


### Write Loop

Next, decide on the type of loop that will work best for your goal of running a calculation on each item of a list.

In this case, you want to convert each item in a list from inches to millimeters (recall than 1 inch = 25.4 mm). So you have a fixed list of values upon which you want to iterate a calculation.

Think about whether a while or for loop would work better for your task.

Which type of loop structure is used in the code below?

# Convert each item in list from in to mm
for month in avg_monthly_precip_in:
month *= 25.4
print(month)

17.779999999999998
19.049999999999997
46.99
74.422
77.46999999999998
51.308
49.022
41.148
46.736
33.274
35.306
21.336


### Expand Loop To Add Results to New List

In the loop above, each month’s value is converted from inches to millimeters and the value is printed; however, the new value is not actually captured anywhere, as the original list is not updated.

You can expand the loop with more code, so that each converted value is actually added to a new list.

Previously in the textbook, you learned how to append items to the end of an existing list using listname += [value], which employs an assignment operator to add the new values to the end of an existing list.

You can add do this with your loop with only two new lines of code:

1. First, you create an empty list that will receive new values using listname = [].
2. Then, you can add a new line of code to append each value after it is calculated using listname += [value].
# Create new empty to receive values
avg_monthly_precip_mm = []

# Convert each item from in to mm and add to new list
for month in avg_monthly_precip_in:
month *= 25.4
avg_monthly_precip_mm += [month]


You can print the values in both lists to see that the original list has not changed, and that the new list contains the converted values.

# Print original list in inches
print(avg_monthly_precip_in)

# Print new list after loop is complete
print(avg_monthly_precip_mm)

[0.7, 0.75, 1.85, 2.93, 3.05, 2.02, 1.93, 1.62, 1.84, 1.31, 1.39, 0.84]
[17.779999999999998, 19.049999999999997, 46.99, 74.422, 77.46999999999998, 51.308, 49.022, 41.148, 46.736, 33.274, 35.306, 21.336]


### Review the List Being Iterated Upon and the Placeholder in Loop

Look carefully at how the variables avg_monthly_precip_mm and month are created.

The list variable avg_monthly_precip_mm was explicitly created; in this case, you manually created the variable avg_monthly_precip_mm as an empty list.

The variable month is the placeholder variable, meaning that it was not explicitly created by you.

Rather, it is created as part of the loop and serves as a placeholder to represent each item from the original list (avg_monthly_precip_in), as the loop iterates.

At the end of the loop, the placeholder variable is equal to the last value that it was assigned (e.g. month is equal to 21.336 when the loop ends).

# Final value of month
month

21.336


## Automate Summary Statistics on Multiple Numpy Arrays

By now, you may be excited that you can automate these kinds of tasks, but you may also be thinking that you would prefer to iterate on numpy arrays or pandas dataframes, instead of working with data values in lists.

You can do that, too! For example, you can build a loop that will calculate summary statistics (such as the sum or median values) of multiple data structures, such as numpy arrays.

Recall that you can use the functions np.sum() and np.median() to calculate sum and median values of a numpy array.

Begin by creating two numpy arrays containing the average monthly precipitation values in 2002 and 2013 for Boulder, Colorado, provided by the U.S. National Oceanic and Atmospheric Administration (NOAA).

MonthPrecipitation (inches) in 2002Precipitation (inches) in 2013
Jan1.070.27
Feb0.441.13
Mar1.501.72
Apr0.204.14
May3.202.66
June1.180.61
July0.091.03
Aug1.441.40
Sept1.5218.16
Oct2.442.24
Nov0.780.29
Dec0.020.50
# Import necessary packages
import numpy as np

# Array of average monthly precip (inches) for 2002 in Boulder, CO
precip_2002_arr = np.array([1.07, 0.44, 1.50, 0.20, 3.20, 1.18,
0.09, 1.44, 1.52, 2.44, 0.78, 0.02])

# Array of average monthly precip (inches) for 2013 in Boulder, CO
precip_2013_arr = np.array([0.27, 1.13, 1.72, 4.14, 2.66, 0.61,
1.03, 1.40, 18.16, 2.24, 0.29, 0.50])


### Create List of Numpy Arrays For Loop

Just like in the previous example, begin by creating the list upon which your loop will iterate.

As you want to iterate on multiple numpy arrays, you can create a list that contains the object names for all of the numpy arrays that you want to work with in the loop.

# Create list of numpy arrays
arr_list = [precip_2002_arr, precip_2013_arr]


### Write Loop

Again, think about what type of loop would work best for this data. This time, you have a list of two object names, upon which you want to iterate a calculation: the numpy arrays for 2002 and 2013.

Which type of loop structure is used in the code below?

# Calculate sum and median for each numpy array in list
for arr in arr_list:
arr_sum = np.sum(arr)
print("sum:", arr_sum)

arr_median = np.median(arr)
print("median:", arr_median)

sum: 13.879999999999999
median: 1.125
sum: 34.15
median: 1.265


Again, you can capture these values in new, separate lists by defining empty lists and using the assignment operator (listname += [value]) to add the results to each list.

# Create new empty lists to receive values
monthly_precip_sum = []
monthly_precip_median = []

# Calculate sum and median for each numpy array and add to new lists
for arr in arr_list:
arr_sum = np.sum(arr)
monthly_precip_sum += [arr_sum]

arr_median = np.median(arr)
monthly_precip_median += [arr_median]


### Review the List Being Iterated Upon and the Placeholder in Loop

In the example above, you explicitly created both monthly_precip_sum and monthly_precip_median as empty lists to which the loop results could be appended.

So at the end of the loop, they are no longer empty, as they have been populated with the results of each iteration of the loop.

# Lists contain the calculated values
print(monthly_precip_sum)
print(monthly_precip_median)

[13.879999999999999, 34.15]
[1.125, 1.265]


The variable arr is the placeholder variable that is created as part of the loop and serves as a placeholder to represent each item from the original list (arr_list), as the loop iterates.

At the end of the loop, arr is equal to the last value that it was assigned (e.g. precip_2013_arr, the last array in the list).

Similarly, at the end of the loop, arr_sum and arr_median are also equal to the last value that was calculated for each (e.g. the sum and median values for precip_2013_arr.

# Final value of arr
print(arr)

# Final value of arr_sum
print(arr_sum)

# Final value of arr_median
print(arr_median)

[ 0.27  1.13  1.72  4.14  2.66  0.61  1.03  1.4  18.16  2.24  0.29  0.5 ]
34.15
1.265


## Automate Calculation on Multiple Columns in Pandas Dataframe

In addition to running a loop on multiple data structures (e.g multiple numpy arrays like in the previous example), you can also run loops on multiple columns of a pandas dataframe.

For example, you may need to convert the measurement units of multiple columns, such as converting the precipitation values from inches to millimeters (1 inch = 25.4 millimeters).

Begin by creating a new pandas dataframe of the same average monthly precipitation values in 2002 and 2013 for Boulder, CO.

# Import necessary packages
import pandas as pd

# Average monthly precip (inches) in 2002 and 2013 for Boulder, CO
precip_2002_2013_df = pd.DataFrame(columns=["month", "precip_2002", "precip_2013"],
data=[
["Jan", 1.07, 0.27],   ["Feb", 0.44, 1.13],
["Mar", 1.50, 1.72],   ["Apr", 0.20, 4.14],
["May", 3.20, 2.66],   ["June", 1.18, 0.61],
["July", 0.09, 1.03],  ["Aug", 1.44, 1.40],
["Sept", 1.52, 18.16], ["Oct", 2.44, 2.24],
["Nov", 0.78, 0.29],   ["Dec", 0.02, 0.50]
])

precip_2002_2013_df

monthprecip_2002precip_2013
0Jan1.070.27
1Feb0.441.13
2Mar1.501.72
3Apr0.204.14
4May3.202.66
5June1.180.61
6July0.091.03
7Aug1.441.40
8Sept1.5218.16
9Oct2.442.24
10Nov0.780.29
11Dec0.020.50

### Create List of Column Names

Just like in the previous examples, begin by creating the list upon which your loop will iterate.

As you want to iterate on multiple columns in a pandas dataframe, you can create a list that contains the column names that you want to work with in the loop.

# Create a list of column names
cols = ["precip_2002", "precip_2013"]


### Write Loop

Once again, think about what type of loop would work best for this data. You have two columns in one pandas dataframe upon which you want to iterate a calculation: 2002 and 2013.

Recall from previous chapters that you can use assignment operators recalculate columns in pandas dataframe:

df["column_name"] *= 25.4

What do you notice about the syntax below that is a little different?

# Convert values for each column in cols list
for column in cols:
precip_2002_2013_df[column] *= 25.4

# Print new values
precip_2002_2013_df

monthprecip_2002precip_2013
0Jan27.1786.858
1Feb11.17628.702
2Mar38.10043.688
3Apr5.080105.156
4May81.28067.564
5June29.97215.494
6July2.28626.162
7Aug36.57635.560
8Sept38.608461.264
9Oct61.97656.896
10Nov19.8127.366
11Dec0.50812.700

### Review the List Being Iterated Upon and the Placeholder in Loop

Note that because column is an implicit variable or placeholder for the columns in the list, you do not need to use quotations "" to indicate a specific column name in the loop such as "precip_2002".

In the first iteration, column would contain the values in the precip_2002 column, while in the last iteration, column would contain the values in the precip_2013 column.

You know you are using an implicit variable because the column name will change with each iteration.

Also, notice the placement of code precip_2002_2013 to display the dataframe after the loop is completed.

This code is not contained with the loop, so you do not see the dataframe each time that the loop iterates. You only see the dataframe when the loop is completed.

Imagine that you have multiple URLs from which you need to download data for a workflow. Rather than writing out the same code to download each file at time, you can use a loop to download all of these files using one set of code.

Begin by importing the necessary package, earthpy, which is needed to access the get_data() function. You will also use os to print the contents of the default data directory.

# Import necessary packages
import os
import earthpy as et


### Create List of URLs For Loop

Just like in the previous examples, begin by creating the list upon which your loop will iterate.

As you want to iterate on multiple URLs, you can create a list that contains the URLs for all of the files that you want to download.

In this case, it is useful to create variables for the individual URLs first, so that you can easily manage them as well as make the code more readable.

# URL for avg monthly precip (inches) for Boulder, CO

# URL for precip data for 2002 and 2013 (inches) for array

# Create list of URLs
urls = [avg_month_precip_url, precip_2002_2013_url]


### Write Loop

Once again, think about what type of loop would work best for this task. You have a list of URLs upon which you want to iterate some code, which in this case is et.data.get_data() to download each file.

# Download each url in list
for file_url in urls:
et.data.get_data(url=file_url)

Downloading from https://ndownloader.figshare.com/files/12565616


### Review the List Being Iterated Upon and the Placeholder in Loop

Note that in order for et.data.get_data() to execute successfully, you must specify that the parameter url for the function is equal to the placeholder, which in this example is file_url.

This is a specific requirement of this function, as et.data.get_data(url) will result in an error that url is not a valid key for a dataset in earthpy (see more details in the code examples for earthpy).

KeyError: "Key not found in earthpy.io.DATA_URLS


With the correct syntax shown in the example above, the loop will execute et.data.get_data(url=file_url) successfully on the URLs provided in the list.

In the first iteration, file_url is set to avg_month_precip_url, and then in the last iteration, file_url is set to precip_2002_2013_url.

### Check Files in Directory

You can see that when using et.data.get_data() in a loop, you no longer get the path printed for each downloaded file.

However, you can use another function from the os package to list the contents (i.e. files and subdirectories) of a directory: os.listdir().

Recall that by default, earthpy downloads files to a subdirectory called earthpy-downloads under the data directory in the earth-analytics directory (e.g. earth-analytics/data/earthpy-downloads/).

With this knowledge, you can define a path to this directory and provide that path to the function os.listdir() to list out the contents of that directory.

The files that you downloaded with the loop above will be listed in the contents of the directory.

# Create path for data directory
data_dir = os.path.join(et.io.HOME, "earth-analytics",

os.listdir(data_dir)

['monthly-precip-2002-2013.csv',
'precip-2002-2013-months-seasons.csv',
'avg-monthly-precip.txt']


Congratulations - you have automated your first tasks in this textbook using Python!

In the next chapter, you will learn how to write custom functions in Python, so that you can customize your code as needed to reduce repetition and automate tasks even further.

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